Memory devices having electrodes comprising nanowires, systems including same and methods of forming same

ABSTRACT

Memory devices having memory cells comprising variable resistance material include an electrode comprising a single nanowire. Various methods may be used to form such memory devices, and such methods may comprise establishing contact between one end of a single nanowire and a volume of variable resistance material in a memory cell. Electronic systems include such memory devices.

FIELD OF THE INVENTION

The present invention relates to methods of forming small electrodes foruse in memory cells of non-volatile memory devices including, forexample, resistance memory devices and phase change memory devices, tomemory devices formed by such methods, and to systems including suchmemory devices.

BACKGROUND OF THE INVEN TION

Various types of non-volatile memory devices employ materials that canbe caused to selectively exhibit more than one value of electricalresistivity. To form a single memory cell (i.e., one bit), a volume ofsuch a material may be provided between two electrodes. A selectedvoltage (or current) may be applied between the electrodes, and theresulting electrical current (or voltage) therebetween will be at leastpartially a function of the particular value of the electricalresistivity exhibited by the material between the electrodes. Arelatively higher electrical resistivity may be used to represent a “1”in binary code, and a relatively low electrical resistivity may be usedto represent a “0” in binary code, or vice versa. By selectively causingthe material between the electrodes to exhibit relatively high and lowvalues of electrical resistivity, the memory cell can be selectivelycharacterized as exhibiting either a “1” or a “0” value.

One particular type of such non-volatile memory devices is the phasechange memory device. In a phase change memory device, the materialsprovided between the electrodes typically are capable of exhibiting atleast two microstructural phases or states, each of which exhibits adifferent value of electrical resistivity. For example, the so-called“phase change material” may be capable of existing in a crystallinephase (i.e., the atoms of the material exhibit relative long-rangeorder) and an amorphous phase (i.e., the atoms of the material do notexhibit any or relatively little long-range order). Typically, theamorphous phase is formed by heating at least a portion of the phasechange material to a temperature above the melting point thereof, andthen rapidly quenching (i.e., cooling) the phase change material tocause the material to solidify before the atoms thereof can assume anylong-range order. To transform the phase change material from theamorphous phase to a crystalline phase, the phase change material istypically heated to an elevated temperature below the melting point, butabove a crystallization temperature, for a time sufficient to allow theatoms of the material to assume the relatively long-range orderassociated with the crystalline phase. For example, Ge₂Sb₂Te₅ (oftenreferred to as “GST”) is often used as a phase change material. Thismaterial has a melting point of about 620° C., and is capable ofexisting in amorphous and crystalline states. To form the amorphous(high resistivity) phase, at least a portion of the material is heatedto a temperature above the melting point thereof by applying arelatively high current through the material between the electrodes (theheat being generated due to the electrical resistance of the phasechange material) for as little as 10 to 100 nanoseconds. As the GSTmaterial quickly cools when the current is interrupted, the atoms of theGST do not have sufficient time to form an ordered crystalline state,and the amorphous phase of the GST material is formed. To form thecrystalline (low resistivity) phase, at least a portion of the materialmay be heated to a temperature of about 550° C., which is above thecrystallization temperature and near, but below, the melting point ofthe GST material, by applying a relatively lower current through the GSTmaterial between the electrodes for a sufficient amount of time (e.g.,as little as about 30 nanoseconds) to allow the atoms of the GSTmaterial to assume the long-range order associated with the crystallinephase, after which the current flowing through the material may beinterrupted. The current passed through the phase change material tocause a phase change therein may be referred to as the “programmingcurrent.”

Various memory devices having memory cells comprising variableresistance material, as well as methods for forming such memory devicesand using such memory devices are known in the art. For example, memorycells comprising variable resistance materials and methods of formingsuch memory cells are disclosed in U.S. Pat. No. 6,150,253 to Doan etal., U. S. Pat. No. 6,294,452, United States Patent ApplicantPublication No. 2006/0034116 A1 to Lam et al., U.S. Pat. No. 7,057,923to Furkay et al., United States Patent Applicant Publication No.2006/0138393 A1 to Seo et al., and United States Patent ApplicantPublication No. 2006/0152186 A1 to Suh et al., the disclosure of each ofwhich is incorporated herein in its entirety by this reference.Furthermore, supporting circuitry that may be used to form a memorydevice comprising memory cells having a variable resistance material, aswell as methods of operating such memory devices, are disclosed in, forexample, United States Patent Applicant Publication No. 2005/0041464 A1to Cho et al., U.S. Pat. No. 7,050,328 to Khouri et al., and U.S. Pat.No. 7,130,214 to Lee, the disclosure of each of which is alsoincorporated herein in its entirety by this reference.

As previously mentioned, the heat generated in a finite volume of thephase change material as the programming current is passed through thevolume of material is due to the electrical resistance of the material.Furthermore, the amount of heat generated in the finite volume of thephase change material is at least partially a function of the currentdensity in the finite volume of phase change material. For a givencurrent passing through a phase change material between two electrodes,the current density in the phase change material is at least partially afunction of the size (e.g., cross-sectional area) of the smallestelectrode. As a result, it is desirable to decrease the size of at leastone of the electrodes such that the current density in the phase changematerial is increased, and the programming current required to cause aphase change in the phase change material is reduced. By decreasing therequired programming current, the energy required to operate the memorydevice may be decreased. Therefore, there is a need for methods that canbe used to form variable resistance memory devices having relativelysmaller electrodes than those presently known in the art.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a partial cross-sectional schematic view of an embodiment ofa memory device of the present invention illustrating three memory cellstherein.

FIGS. 1B and 1C show the electrodes and variable resistance material ofone memory cell shown in FIG. 1A and are used to illustrate one mannerof operation thereof.

FIGS. 2A-2I are partial cross-sectional side views of a workpiece andillustrate a first embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A and thatincludes using a shadow mask deposition process to form a catalyticstructure.

FIGS. 3A-3F are partial cross-sectional side views of a workpiece andillustrate a second embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A.

FIGS. 4A-4D are partial cross-sectional side views of a workpiece andillustrate a third embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A.

FIGS. 5A-5F are partial cross-sectional side views of a workpiece andillustrate a fourth embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A.

FIGS. 6A-6I are partial cross-sectional side views of a workpiece andillustrate a fifth embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A.

FIGS. 7A-7I are partial cross-sectional side views of a workpiece andillustrate a sixth embodiment of a method of the present invention thatmay be used to form a memory device like that shown in FIG. 1A.

FIGS. 8A-8E are partial cross-sectional side views of a workpiece andillustrate a seventh embodiment of a method of the present inventionthat may be used to form a memory device like that shown in FIG. 1A.

FIG. 9 is a schematic block diagram illustrating one embodiment of anelectronic system of the present invention that includes a memory deviceas shown in FIG. 1A.

DETAILED DESCRIPTION OF THE INVENTION

As discussed in further below, in some embodiments, the presentinvention comprises memory devices having a volume of variableresistance material disposed between two electrodes. At least one of theelectrodes is or includes a single nanowire having one end in electricalcontact with the volume of variable resistance material and a second endin electrical contact with other conductive features or elements of thememory device. In additional embodiments, the present inventioncomprises electronic systems that include one or more such memorydevices. The one or more such memory devices may be in electricalcommunication with an electronic signal processor. In other embodiments,the present invention includes methods of forming such memory devices.Such methods may include providing contact between one end of a singlenanowire and a volume of variable resistance material.

As used herein, the term “variable resistance material” means anymaterial capable of exhibiting more than one value of electricalresistivity, and hence, conductivity. Variable resistance materials mayinclude, for example, phase change materials (e.g., chalcogenides suchas, for example Ge₂Sb₂Te₅, Te₈₁Ge₁₅Sb₂S₂, and Sb₂Te₃), colossal magnetresistive films (e.g., Pr_((1-x))Ca_(x)MnO₃ (PCMO), La_((1-x))Ca_(x)MnO₃ (LCMO), and Ba_((1-x ))Sr_(x)TiO₃), oxide materials(e.g., doped or undoped binary or ternary oxides such as, for example,Al₂O₃, BaTiO₃, SrTiO₃, Nb₂O₅, SrZrO₃, TiO₂, Ta₂O₅, NiO, ZrO_(x),HfO_(x), and Cu₂O), which may have a Perovskite structure, and materialshaving the general formula A_(x)B_(y), where B is selected from sulfur(S), selenium (Se), and tellurium (Te), and mixtures thereof, and whereA includes at least one element from Group III-B (B, Al, Ga, In, Tl),Group IV-B (C, Si, Ge, Sn, Pb), Group V-B (N, P, As, Sb, Bi), or GroupVII-B (F, Cl, Br, I, At) with one or more dopants selected from noblemetal and transition metal elements such as, for example, Au, Ag, Pt,Cu, Cd, In, Ru, Co, Cr, Ni, Mn, and Mo.

As used herein, the term “nanowire” means any elongated structure havingtransverse cross-sectional dimensions averaging less than about 50nanometers.

As used herein, the term “superlattice structure” means a structurepredominantly comprised of periodically alternating layers of differentmaterials.

As used herein, the term “III-V type semiconductor material” means anymaterial predominantly comprised of one or more elements from GroupIII-B of the periodic table (B, Al, Ga, In, and Ti) and one or moreelements from Group V-B of the periodic table (N, P, As, Sb, and Bi).

As used herein, the term “II-VI type semiconductor material” means anymaterial predominantly comprised of one or more elements from Group II-Bof the periodic table (Zn, Cd, and Hg) and one or more elements fromGroup VI-B of the periodic table (O, S, Se, Te, and Po).

As used herein, the term “wafer” means any structure that includes alayer of semiconductor type material including, for example, silicon,germanium, gallium arsenide, indium phosphide, and other III-V or II-VItype semiconductor materials. Wafers include, for example, not onlyconventional wafers but also other bulk semiconductor substrates suchas, by way of nonlimiting example, silicon-on-insulator (SOI) typesubstrates, silicon-on-sapphire (SOS) type substrates, and epitaxiallayers of silicon supported by a layer of base material. Semiconductortype materials may be doped or undoped. Furthermore, when reference ismade to a “wafer” in the following description, previous process stepsmay have been utilized to at least partially form elements or componentsof a circuit or device in or over a surface of the wafer.

The illustrations presented herein are not meant to be actual views ofany particular memory device, memory cell, workpiece, or system, but aremerely idealized representations that are employed to describe thepresent invention. Additionally, elements common between figures mayretain the same numerical designation.

FIG. 1A is a partial cross-sectional schematic view of an embodiment ofa memory device 10 of the present invention. The memory device 10 mayinclude an integrated circuit comprising a plurality of memory cells 12,and the memory cells 12 may be arranged in an array on or in a substrate11. By way of example and not limitation, the memory cells 12 may bearranged in a plurality of rows and columns. FIG. 1A is a partialcross-sectional view taken vertically through the substrate 11 andillustrates three memory cells 12 in a common row or column of the arrayof memory cells 12.

To facilitate illustration, the memory cells 12 are shown in FIG. 1A asoccupying a major vertical portion of the substrate 11. It isunderstood, however, that in actuality, the substrate 11 may berelatively thicker than illustrated, and the memory cells 12 may occupya relatively thinner portion of the substrate 11. Furthermore, onlyactive elements of the memory cells 12 (i.e., the elements of the memorycells 12 through which charge carriers travel), or materials used toform such active elements, are cross-hatched to simplify thecross-sectional figures herein.

The substrate 11 may comprise, for example, a material such as glass orsapphire, or the substrate may comprise a full or partial wafer, whichmay facilitate processing using conventional semiconductor fabricationprocesses.

As shown in FIG. 1A, each memory cell 12 may comprise a first electrode16, a second electrode 18, and a volume of variable resistance material20 disposed between the first electrode 16 and the second electrode 18.

In some embodiments, the variable resistance material 20 may comprise aphase change material. For example, the variable resistance material 20may comprise a phase change material such as a chalcogenide material.Typical chalcogenide materials are alloys predominantly comprisingtellurium (Te), germanium (Ge), and antimony (Sb) and include, forexample, Ge₂Sb₂Te₅, Te₈₁Ge₁₅Sb₂S₂, and Sb₂Te₃ Chalcognide materials maybe characterized by the general chemical formulaTe_(a)Ge_(b)Sb_(100(a+b)), where “a” is less than about eighty-five (85)and “b⇄ is above about eight (8).

In additional embodiments, the variable resistance material 20 maycomprise one of various materials used to form so-called “colossalmagnetoresistive films” such as, for example, Pr_((1-x ))Ca_(x)MnO₃(PCMO), La_((1-x ))Ca_(x)MnO₃ (LCMO), and Ba_((1-x ))Sr_(x)TiO₃. In yetother embodiments, the variable resistance material 20 may comprise abinary or ternary doped or undoped oxide material such as, for example,Al₂O₃, BaTiO₃, SrTiO₃, Nb₂O₅, SrZrO₃, TiO₂, Ta₂O₅, NiO, ZrO_(x),HfO_(x), and Cu₂O. Furthermore, the variable resistance material 20 mayhave a Perovskite structure. Yet another type of variable resistancematerial includes a doped chalcogenide glass of the general formulaA_(x)B_(y), where B is selected from sulfur (S), selenium (Se), andtellurium (Te), and mixtures thereof, and where A includes at least oneelement from Group III-B (B, Al, Ga, In, Tl), Group IV-B (C, Si, Ge, Sn,Pb), Group V-B (N, P, As, Sb, Bi), or Group VII-B (F, Cl, Br, I, At)with one or more dopants selected from noble metal and transition metalelements such as, for example, Au, Ag, Pt, Cu, Cd, In, Ru, Co, Cr, Ni,Mn, and Mo.

The first electrode 16 of each memory cell 12 may comprise a singlenanowire 22 having a first end 24 proximate to or in direct physicalcontact with a surface of the volume of variable resistance material 20and a second end 26 structurally and electrically coupled to otherconductive features of the memory device 10. For example, the firstelectrode 16 of each memory cell 12 may further comprise a conductivepad 28, and the second end 26 of the single nanowire 22 may bestructurally and electrically coupled to the conductive pad 28. In someembodiments, each conductive pad 28 may comprise a discrete, laterallyisolated volume of conductive material, as shown in FIG. 1A. In otherembodiments, each conductive pad 28 may simply comprise an area orregion of an elongated laterally extending conductive trace.

By way of example and not limitation, the single nanowire 22 of eachmemory cell 12 may comprise a nanotube, such as a single wall carbonnanotube (SWCNT) or a multi-walled carbon nanotube (MWCNT). Inadditional embodiments, each nanowire 22 may comprise a substantiallysolid nanowire substantially comprised of a semiconductor material suchas, for example, silicon, germanium, gallium, a III-V type semiconductormaterial, or a II-VI type semiconductor material. Such nanowires 22optionally may have an integrated PN junction or a superlatticestructure. Furthermore, each nanowire 22 may comprise a single crystal.In yet other embodiments, each nanowire 22 may comprise a substantiallysolid nanowire substantially comprised of a metal such as, for example,cobalt, copper, gold, nickel, platinum, or silver. Any type of nanowire22 may be used as long as the nanowire exhibits sufficient electricalconductivity and can be formed, grown, placed, or otherwise providedwithin the memory cells 12, as discussed in further detail below.

With continued reference to FIG. 1A, the second end 26 of each nanowire22 may be indirectly structurally and electrically coupled with theconductive pad 28 by way of a conductive catalytic structure 30. Inother words, a conductive catalytic structure 30 may be disposed betweenthe second end 26 of each nanowire 22 and the conductive pad 28, and theconductive catalytic structure 30 may be structurally and electricallycoupled to both the nanowire 22 and the conductive pad 28. Theconductive catalytic structures 30 may be used to catalyze the formationof the single nanowires 22 of each memory cell 12, as discussed infurther detail below.

In some embodiments, each nanowire 22 may be grown or otherwise formedin situ, while in other embodiments, each nanowire 22 may be grown orformed elsewhere and subsequently positioned within a memory cell 12, asdiscussed in further detail below.

In some embodiments, each nanowire 22 may have an average diameter ofless than about ten nanometers (10 nm). More particularly, each nanowiremay have an average diameter of between about three nanometers (3 nm)and about six nanometers (6 nm) in some embodiments. Even moreparticularly, each nanowire may have an average diameter of betweenabout four nanometers (4 nm) and about five nanometers (5 nm) in someembodiments.

The average thickness of the volume of variable resistance material 20between the first end 24 of each nanowire 22 and the second electrode 18may be between about one and about three times the average diameter ofeach nanowire 22. In some embodiments, the average thickness of eachvolume of variable resistance material 20 between the first end 24 ofeach nanowire 22 and the second electrode 18 may be about twice theaverage diameter of each nanowire 22.

The second electrode 18 of each memory cell 12 may be substantiallysimilar to the conductive pads 28 of the first electrodes 16 and maycomprise a discrete, laterally isolated volume of conductive materialsuch as a metal. In other embodiments, each second electrode 18 maysimply comprise an area or region of an elongated laterally extendingconductive trace.

In some embodiments, each second electrode 18 may communicateelectrically with a conductive line 34 by way of electrical contacts 35,and each first electrode 16 also may communicate electrically withanother conductive line 36 by way of electrical contacts 37. Inadditional embodiments, the second electrodes 18 may simply comprise aregion or portion of a conductive line, and the memory cells 12 need notinclude a separate conductive line 34 and electrical contacts 35.Similarly, in additional embodiments, the conductive pads 28 of thefirst electrodes 16 also may comprise a region or portion of aconductive line, and the memory cells 12 need not include a separateconductive line 36 and electrical contacts 37.

Furthermore, in additional embodiments, the first electrode 16 and thesecond electrode 18 may not each electrically communicate with aconductive line, and one or both of the first electrode 16 and thesecond electrode 18 may simply communicate with a conductive pad.

Although not shown in FIG. 1A, each memory cell 12 also may include anaccess transistor for selectively accessing the same for read and writeoperations, as known in the art.

A manner in which the memory cell 12 may be used or characterized so asto represent either a “0” or a “1” in binary code is briefly describedbelow with reference to FIGS. 1B and 1C.

FIG. 1B is an enlarged view of the first electrode 16, second electrode18, and variable resistance material 20 of one memory cell 12 shown inFIG. 1A. As previously discussed, the variable resistance material 20may comprise a phase change material. The variable resistance material20 of the memory cell 12 shown in FIG. 1B may exist in a first state orphase (i.e., the atoms may be disposed in a particular microstructure),which can be detected by providing a relatively low voltage between thefirst electrode 16 and the second electrode 18 and measuring themagnitude (e.g., amps) of the resulting current passing between thefirst electrode 16 and the second electrode 18 through the variableresistance material 20. By way of example and not limitation, this firststate or phase (and, hence, the current magnitude) may be selected torepresent a “1” in binary code.

To change the state or phase of the variable resistance material 20, arelatively high voltage may be provided between the first electrode 16and the second electrode 18 to induce a relatively high current throughthe variable resistance material 20. This relatively high currentflowing through the variable resistance material 20 may be referred toas the programming current and is used to heat at least a small portion21 of the volume of variable resistance material 20 to a sufficienttemperature to cause a change in the state or phase of the portion 21 ofthe variable resistance material 20, as shown in FIG. 1C. The portion 21of the variable resistance material 20 then may exhibit an electricalresistivity (and, inversely, a conductivity) in the second state orphase that differs from the electrical resistivity in the first state orphase. As a result, the second state or phase can be detected by againproviding a relatively low voltage between the first electrode 16 andthe second electrode 18 and measuring the magnitude (e.g., amps) of theresulting current passing between the first electrode 16 and the secondelectrode 18, which will be different from the magnitude of the measuredcurrent when the variable resistance material 20 is in the first stateor phase. By way of example and not limitation, this second state orphase (and, hence, the second current magnitude) may be selected torepresent a “0” in binary code.

The heat generated in the portion 21 of the variable resistance material20 as the programming current is passed therethrough is due to theelectrical resistance of the variable resistance material 20.Furthermore, the amount of heat generated in the portion 21 of thevariable resistance material 20 is at least partially a function of thecurrent density in the portion 21 of the variable resistance material20. For a given current passing through the variable resistance material20 between the first electrode 16 and the second electrode 18, thecurrent density in the variable resistance material 20 is at leastpartially a function of the size of the smaller of the electrodes 16,18. By using the first end 24 of a single nanowire 22 as the portion ofthe first electrode 16 that is immediately adjacent to the volume ofvariable resistance material 20, the current density in the portion 21of the variable resistance material 20 is increased, and the programmingcurrent required to cause a phase change in the portion 21 of thevariable resistance material 20 is reduced. By decreasing the requiredprogramming current, the energy required to operate the memory device 10may be decreased. As a result, memory devices 10 of the presentinvention may be operated using less power relative to memory devicespresently known in the art, they may be operated at higher speedsrelative to memory devices presently known in the art, or may offer bothsuch advantages.

Various methods for forming embodiments of memory devices according tothe present invention, such as the memory device 10 shown in FIG. 1A,are described below. To facilitate description, the methods aredescribed with reference to a single memory cell 12. In practice,however, a plurality of memory cells 12 may be formed substantiallysimultaneously on a substrate 11, and the memory cells 12 may comprisememory cells 12 of one or a plurality of memory devices 10.

A first embodiment of a method that may be used to form the memorydevice 10 shown in FIG. 1A is described with reference to FIGS. 2A-2I.Referring to FIG. 2A, a substrate 11 may be provided, which, aspreviously discussed, may comprise a full or partial wafer ofsemiconductor material or a material such as glass or sapphire. Aplurality of conductive pads 28 may be formed on or in a surface of thesubstrate 11 to form a workpiece, as shown in FIG. 2B. The conductivepads 28 may comprise, for example, a conductive metal such as tungstenor titanium nitride, and may be formed using, for example, metal layerdeposition techniques (e.g., chemical vapor deposition (CVD), physicalvapor deposition (PVD), sputtering, thermal evaporation, or plating) andpatterning techniques (e.g., masking and etching) known in the art ofintegrated circuit fabrication. Additional features, such as, forexample, the conductive lines 36 (which may simply comprise conductivepads in additional embodiments) and electrical contacts 37 (FIG. 1A)also may be formed on or in the surface of the substrate 11 in a similarmanner (prior and/or subsequent to forming the conductive pads 28),although such additional features are not illustrated in FIGS. 2A-2I tosimplify the figures.

As shown in FIG. 2B, a layer of dielectric material 40 may be providedover the workpiece (i.e., an exposed major surface of the substrate 11and the conductive pad 28), and a mask layer 42 may be provided over thelayer of dielectric material 40. By way of example and not limitation,the layer of dielectric material 40 may comprise an oxide such as silica(SiO₂) or silicon nitride (Si₃N₄), and may be formed by chemical vapordeposition, by decomposing tetraethyl orthosilicate (TEOS), or by anyother process known in the art of integrated circuit fabrication. Themask layer 42 may comprise, for example, a layer of photoresist materialor a layer of metal material. An aperture or via 44 then may be formedby patterning the mask layer 42 to form an opening therein at thelocation at which it is desired to form the via 44, and etching thelayer of dielectric material 40 through the aperture in the mask layer42 using, for example, an anisotropic reactive ion (i.e., plasma)etching process, to expose the underlying conductive pad 28. Theparticular composition of the gases used to generate the reactive ionsand the operating parameters of the etching process may be selectedbased on the composition of the layer of dielectric material 40, themask layer 42, and the conductive pad 28.

Referring to FIG. 2C, after forming the via 44 over the underlyingconductive pad 28, another etchant that selectively etches away thelayer of dielectric material 40 at a faster rate than the mask layer 42and the conductive pad 28 may be used to etch away the exposed surfacesof the layer of dielectric material 40 within the via 44, so as toundercut the via 44. By way of example and not limitation, an isotropicwet chemical etching process may be used to undercut the via 44. Again,the particular composition of the chemical etchant may be selected basedon the composition of the layer of dielectric material 40, the masklayer 42, and the conductive pad 28.

In additional embodiments, the via 44 may be formed using a singleisotropic wet chemical etching process instead of a separate anisotropicreactive ion etching process followed by an isotropic wet chemicaletching process.

Referring to FIG. 2D, a shadow deposition process may be used to form acatalytic structure 30 on the conductive pad 28 within the via 44. Suchprocesses are described in, for example, United States PatentApplication Publication No. US 2006/0131556 A1, which was published Jun.22, 2006 and entitled “Small Electrode For Resistance Variable Devices,”the disclosure of which is incorporated herein in its entirety by thisreference. For example, the substrate 11 may be provided in a depositionchamber (not shown), and a general directional flow of atoms of catalystmaterial may be generated therein using, for example, an evaporationprocess or a collimated sputtering process. The general directional flowof atoms of catalyst material is represented in FIG. 2D by thedirectional arrows 48. As shown, the workpiece (or substrate 11) may beoriented at an acute angle of less than ninety degrees (90°) relative tothe general flow of atoms of catalyst material within the depositionchamber, and the workpiece may be rotated in the plane of the substrate11, as indicated by the directional arrow 50, while the atoms ofcatalyst material are deposited thereon. By orienting the workpiece atan angle relative to the general direction of flow of atoms of catalystmaterial and rotating the substrate 11 as the catalyst material isdeposited thereon, a generally conical catalytic structure 30 may beformed on the conductive pad 28 within the via 44. The base of thegenerally conical catalytic structure 30 may be structurally andelectrically coupled to the conductive pad 28 as the catalytic structure30 is formed thereon, and the tip of the generally conical catalyticstructure 30 may have a cross-sectional area similar to, or less than,that of a desired average diameter of a nanowire 22 (FIG. 1A) to beformed, grown, or otherwise provided thereon. During the shadowdeposition process, a layer of catalyst material 52 also may bedeposited over the mask layer 42, as shown in FIG. 2D.

In additional embodiments, the workpiece (or substrate 11) may beoriented substantially perpendicular (i.e., at an angle of about ninetydegrees (90°)) relative to the general flow of atoms of catalystmaterial within the deposition chamber.

After forming the catalytic structure 30 on the conductive pad 28 withinthe via 44 (FIG. 2C), the layer of catalyst material 52 and the masklayer 42 may be removed using, for example, a chemical-mechanicalpolishing (CMP) process, a selective etching process, or a lift-offprocess to form the structure shown in FIG. 2E. For example, a lift-offlayer (not shown) may be formed over the layer of dielectric material40, after which the mask layer 42 may be deposited over the lift-offlayer. The via 44 then may be formed through the mask layer 42, thelift-off layer, and the layer of dielectric material 40, and thecatalytic structure 30 may be formed on the conductive pad 28, which mayresult in formation of the layer of catalyst material 52, as previouslymentioned. The lift-off layer then may be stripped away from theworkpiece, and the overlying mask layer 42 and layer of conductivematerial 52 may be removed from the workpiece together with theunderlying lift-off layer. In additional embodiments, the mask layer 42itself may serve as a lift-off layer.

Referring to FIG. 2F, the remaining portion of the via 44 surroundingthe catalytic structure 30 may be filled with a dielectric material 54,which, optionally, may be substantially identical to the dielectricmaterial 40. By way of example and not limitation, a conformal layer(not shown) of dielectric material 54 may be deposited over theworkpiece (or substrate 11) to a thickness sufficient to fill theremaining portion of the via 44 surrounding the catalytic structure 30.An additional chemical-mechanical polishing (CMP) process then may beused to planarize the surface of the workpiece and to expose the tip 31of the catalytic structure 30 through the dielectric material 54, asshown in FIG. 2F. The chemical-mechanical polishing (CMP) process may beselectively terminated when the area of the surface of the tip 31exposed through the dielectric material 54 by the chemical-mechanicalpolishing (CMP) process reaches a selected predetermined size. By way ofexample and not limitation, the chemical-mechanical polishing (CMP)process may be selectively terminated when the area of the exposedsurface of the tip 31 has a cross-sectional area of less than aboutthree hundred square nanometers (300 nm²).

Referring to FIG. 2G, the tip 31 of the catalytic structure 30 then maybe used to catalyze formation or growth of a single nanowire 22 thereon.Various methods of forming and/or growing nanowires using correspondingcatalyst materials are known in the art and may be used to form thesingle nanowire 22. Some of such methods are described in, for example,Younan Xia et al., “One-Dimensional Nanostructures: Synthesis,Characterization and Applications,” 15 Advanced Materials 353-389 (March2003), the entire disclosure of which is incorporated herein in itsentirety by this reference. By way of example and not limitation,chemical-vapor-deposition processes, which optionally may employ theso-called vapor-liquid-solid (VLS) mechanism, may be used to grow ananowire 22 on the tip of the catalytic structure 30, as known in theart. As one non-limiting example, the catalytic structure 30 maycomprise gold, and the nanowire 22 may comprise a doped silicon (Si).Such a doped silicon nanowire may be formed using a chemical vapordeposition process and the vapor-liquid-solid (VLS) mechanism, as knownin the art. As another non-limiting example, the catalytic structure 30may comprise at least one of Ti, Co, Ni, Au, Ta, polysilicon,silicon-germanium, platinum, iridium, titanium nitride, or tantalumnitride, and the nanowire 22 may comprise iridium oxide (IrO_(x)), asdescribed in United States Patent Publication No. 2006/0086314 A1 toZhang et al., the entire disclosure of which is incorporated herein inits entirety by this reference. Furthermore, as previously discussed,the nanowire may comprise a III-V type semiconductor material or a typesemiconductor material. Various types of semiconductor materials thatmay be used to form nanowires, as well as the reactant precursormaterials and catalyst materials that may be used to catalyze formationof such nanowires are disclosed in United States Patent Publication No.2004/0028812 A1 to Wessels et al., the entire disclosure of which isalso incorporated herein in its entirety by this reference.

In additional embodiments, the nanowire 22 may be fabricated elsewhererather than in situ and positioned within the memory cell 12 using, forexample, a selectively oriented electrical field. In such methods, thecatalytic structure 30 may be replaced with an electrically conductivestructure having a similar shape and configuration to the catalyticstructure 30 but that does not comprise a catalyst material.

As shown in FIG. 2I, in some embodiments, the nanowire 22 may beoriented substantially perpendicular to the plane of the substrate 11.Various techniques for orienting nanowires 22 in a selected directionare known in the art and may be used to orient the nanowire 22substantially perpendicular to the plane of the substrate 11. Forexample, an electrical field may be generated and selectively orientedto cause the nanowire 22 to selectively tailor the orientation of thenanowire 22 as the nanowire 22 is formed or grown on the catalyticstructure 30 or otherwise positioned in the memory cell 12, as describein, for example, Cheng et al., “Role of Electric Field on Formation ofSilicon Nanowires,” J. Applied Physics, Vol. 94, No. 2 (2003), theentire disclosure of which is incorporated herein in its entirety bythis reference.

As shown in FIG. 2H, after using the tip 31 of the catalytic structure30 to catalyze formation or growth of the single nanowire 22 thereon,another layer of dielectric material 56 may be provided around thesingle nanowire 22. By way of example and not limitation, the layer ofdielectric material 56 may comprise a nitride material such as siliconnitride (Si₃N₄). In additional embodiments, the layer of dielectricmaterial 56 may be substantially identical to the layer of dielectricmaterial 40, and may comprise, for example, an oxide material. The layerof dielectric material 56 may be substantially conformal and may bedeposited over the workpiece to a thickness sufficient to substantiallycover the nanowire 22. The layer of dielectric material 56 may beplanarized using a chemical-mechanical polishing (CMP) process to exposethe first end 24 of the nanowire 22 through the dielectric material 56,as shown in FIG. 2H.

Referring to FIG. 2I, after exposing the first end 24 of the nanowire 22through the dielectric material 56, a volume of variable resistancematerial 20 may be provided on the exposed surface of the layer ofdielectric material 56 and over the first end 24 of the nanowire 22, andthe second electrode 18 may be provided over the volume of variableresistance material 20. By way of example and not limitation, a layer ofvariable resistance material 20 may be deposited over the workpiece (orsubstrate 11), and a layer of metal for forming the second electrode 18may be deposited on the layer of variable resistance material 20. Amasking and etching process then may be used to selectively removeregions or areas of both the layer of metal material and the layer ofdielectric material 56 leaving behind the volume of variable resistancematerial 20 over the nanowire 22 and the second electrode 18 over thevolume of variable resistance material 20.

Additional features and elements, such as, for example, conductive lines34 and electrical contacts 35 (FIG. 1A), then may be formed over thelayer of variable resistance material 20 and the second electrode 18 asnecessary or desired.

A second embodiment of a method that may be used to form an embodimentof a memory device 10 is described below with reference to FIGS. 3A-3F.Referring to FIG. 3A, a workpiece may be provided that is substantiallysimilar to the workpiece shown in FIG. 2B and includes the substrate 11,conductive pad 28, layer of dielectric material 40, and mask layer 42.The workpiece shown in FIG. 3A, however, also includes a polish-stoplayer 58 disposed between the layer of dielectric material 40 and themask layer 42. As a non-limiting example, the polish-stop layer 58 maycomprise a layer of silicon nitride (Si₃N₄). To form the workpiece shownin FIG. 3A, the layer of dielectric material 40 may be deposited,followed by the polish-stop layer 58, and the mask layer 42. Thepolish-stop layer 58 may be deposited using, for example, a chemicalvapor deposition (CVD) process. A via 44 may be formed through the layerof dielectric material 40, the polish-stop layer 58, and the mask layer42 to expose the underlying conductive pad 28 using methods identical orsubstantially similar to those previously described in relation to FIG.2B.

Referring to FIG. 3B, after forming the via 44, a catalytic structure 30may be formed on the conductive pad 28 using a shadow deposition processas previously described in relation to FIG. 2D, after which the layer ofcatalyst material 52 and the mask layer 42 may be removed in the mannerpreviously described in relation to FIG. 2E. A substantially conformallayer of dielectric material 54 then may be provided over the workpiece,as shown in FIG. 3C, to fill the regions of the via 44 surrounding thecatalytic structure 30. As shown in FIG. 3D, a chemical-mechanicalpolishing (CMP) process then may be used to remove the portions of thelayer of dielectric material 54 overlying the polish-stop layer 58. Theslurry and polishing pad of the apparatus used to perform thechemical-mechanical polishing (CMP) process may be selectively tailoredso as to wear away the layer of dielectric material 54 at a rate that isfaster than the rate at which the process will wear away the underlyingpolish-stop layer 58. In this manner, substantially all of the layer ofdielectric material 54 overlying the polish-stop layer 58 may be removedfrom the workpiece without completely removing the polish-stop layer 58.The polish-stop layer 58 may be used to ensure that only a selectedamount of the tip 31 of the catalytic structure 30, if any at all, isremoved from the catalytic structure 30 during the chemical-mechanicalpolishing (CMP) process.

Referring to FIG. 3E, the tip 31 of the catalytic structure 30 then maybe used to catalyze formation or growth of a single nanowire 22 thereon,as previously described in relation to FIG. 2G, after which a layer ofdielectric material 56 may be provided around the nanowire 22, as shownin FIG. 3F. As previously discussed, a chemical-mechanical polishing(CMP) process may be used to planarize the layer of dielectric material56 and expose the first end 24 of the nanowire 22 therethrough. Thevolume of variable resistance material 20 then may be provided over thefirst end 24 of the nanowire 22, and the second electrode 18 may beprovided over the volume of variable resistance material 20, usingmethods previously described in relation to FIG. 2I.

A third embodiment of a method that may be used to form an embodiment ofa memory device 10 like that shown in FIG. 1A is described below withreference to FIGS. 4A-4D. Referring to FIG. 4A, a workpiece may beprovided that is substantially similar to the workpiece shown in FIG. 2E(using methods previously described in relation to FIGS. 2A-2E) andincludes the substrate 11, conductive pad 28, layer of dielectricmaterial 40, and a generally conical catalytic structure 30 on theconductive pad 28. After providing the workpiece shown in FIG. 4A, theremaining portions of the layer of dielectric material 40 may be removedby, for example, using an isotropic wet chemical etching process, toform a structure like that shown in FIG. 4B. Referring to FIG. 4C, asubstantially conformal layer of dielectric material 54 then may bedeposited over the workpiece. In some embodiments, the substantiallyconformal layer of dielectric material 54 may have an average thicknessgreater than the distance by which the catalytic structure 30 extendsfrom the surface of the conductive pad 28 and the substrate 11. Achemical-mechanical polishing (CMP) process then may be used toplanarize the layer of dielectric material 54 and expose a selectedportion of the tip 31 of the catalytic structure 30 through thedielectric material 54, as shown in FIG. 4D. After forming the structureshown in FIG. 4D, methods like those previously described in relation toFIGS. 2G-2I may be used to complete the formation of the memory cell 12(FIG. 1A).

A fourth embodiment of a method that may be used to form a memory device10 like that shown in FIG. 1A is described below with reference to FIGS.5A-5F. Referring to FIG. 5A, a workpiece may be provided that issubstantially similar to the workpiece shown in FIG. 4B and includes thesubstrate 11, conductive pad 28, and a generally conical catalyticstructure 30 on the conductive pad 28. Referring to FIG. 5B, asubstantially conformal layer of dielectric material 54 then may bedeposited over the workpiece. The substantially conformal layer ofdielectric material 54 may have an average thickness that is less thanthe distance by which the catalytic structure 30 extends from thesurface of the conductive pad 28 and the substrate 11. In someembodiments, the substantially conformal layer of dielectric material 54may have an average thickness of between about two nanometers (2 nm) andabout fifty nanometers (50 nm).

Referring to FIG. 5C, an anisotropic etching process then may be used toremove the generally laterally extending regions of the layer ofdielectric material 54, including the regions overlying the substrate 11and a portion of the layer of dielectric material 54 on the tip 31 ofthe generally conical catalytic structure 30. After such an anisotropicetching process, only portions of the layer of dielectric material 54 onthe lateral sides of the catalytic structure 30 may remain after theanisotropic etching process, and the tip 31 of the catalytic structure30 may be exposed through the dielectric material 54. The anisotropicetching process may comprise, for example, an anisotropic reactive ion(e.g., plasma) etching process (RIE).

As shown in FIG. 5D, after exposing the tip 31 of the catalyticstructure 30 through the dielectric material 54, growth or formation ofa single nanowire 22 may be catalyzed using the tip 31 of the catalyticstructure 30, as previously described in relation to FIG. 2G. Anotherlayer of dielectric material 56 then may be deposited over the workpieceand around the nanowire 22 and catalytic structure 30. Achemical-mechanical polishing (CMP) process may be used to planarize thesurface of the layer of dielectric material 56 and to expose a selectedportion of the first end 24 of the nanowire 22 therethrough, as shown inFIG. 5E. As shown in FIG. 5F, a volume of variable resistance material20 and a second electrode 18 then may be formed on the workpiece overthe first end 24 of the nanowire 22 in the manner previously describedwith reference to FIGS. 2G-2I.

A fifth embodiment of a method that may be used to form an embodiment ofa memory device 10 like that shown in FIG. 1A is described below withreference to FIGS. 6A-6I. Referring to FIG. 6A, a workpiece may beprovided that includes the substrate 11 and a conductive pad 28. A layerof catalyst material 68 may be deposited over the substrate 11. By wayof example and not limitation, the layer of catalyst material 68 may bedeposited using a physical vapor deposition (PVD) (e.g., sputtering orthermal evaporation) process, a chemical vapor deposition (CVD) process,an electroless deposition process, or by an electroless deposition toform a seed layer followed by an electroplating process. The layer ofcatalyst material 68 may have an average thickness of between aboutfifty nanometers (50 nm) and about five hundred nanometers (500 nm). Amask layer 70 then may be provided over the layer of catalyst material68. The mask layer 70 may comprise, for example, a layer of photoresist,a layer of nitride material (e.g., Si₃N₄), or a layer of oxide material(e.g., SiO₂). The mask layer 70 then may be selectively patterned toform a discrete region 72 of mask material on the surface of the layerof catalyst material 68 overlying the conductive pad 28. By way ofexample and not limitation, the discrete region 72 of mask material maybe generally circular and may have an average diameter of between abouttwenty nanometers (20 nm) and about one hundred nanometers (100 nm).

Referring to FIG. 6C, an anisotropic dry reactive ion (i.e., plasma)etching process then may be used to remove the regions of the layer ofcatalyst material 68 that are not protected by the discrete region 72 ofmask material so as to form a catalytic structure 76. In other words,only a portion of the layer of catalyst material 68 vertically under thediscrete region 72 of mask material may remain after the anisotropicetching process. As previously discussed, the discrete region 72 of maskmaterial may be, for example, generally circular, and the resultingcatalytic structure 76 may be generally cylindrical and may have anaverage diameter substantially similar to the average diameter of thediscrete region 72 of mask material. The discrete region 72 of maskmaterial remaining on the end of the catalytic structure 76 may beremoved from the end of the catalytic structure 76 using, for example, awet chemical etching process.

Referring to FIG. 6D, the end 77 of the catalytic structure 76 oppositethe conductive pad 28 may be sharpened so as to reduce thecross-sectional area of the catalytic structure 76 near the end 77thereof By way of example and not limitation, the end 77 of thecatalytic structure 76 may be sharpened using at least one of ananisotropic reactive ion (i.e., plasma) etching process, a sputteringprocess, and an oxidation process. For example, an anisotropic reactiveion etching process may sharpen the end 77 of the catalytic structure 76due to an increased rate of etching at the relatively sharp edges nearthe end 77 of the catalytic structure 76. As another example, thecatalytic structure 76 may be sharpened using a sputtering process bybombarding the end 77 of the catalytic structure 76 with ions or otherparticles (e.g., argon atoms). As yet another example, the catalyticstructure 76 may be sharpened using an oxidation process by oxidizingthe exterior surfaces of the catalytic structure 76, and subsequentlyremoving the oxide layer formed in the exterior surfaces of thecatalytic structure 76.

Referring to FIG. 6E, a substantially conformal layer of dielectricmaterial 54 then may be deposited over the workpiece and around thecatalytic structure 76. The substantially conformal layer of dielectricmaterial 54 may have an average thickness that is greater than thedistance by which the catalytic structure 76 extends from the surface ofthe conductive pad 28 and the substrate 11. In this configuration, thecatalytic structure 76 may be substantially buried within the dielectricmaterial 54.

As shown in FIG. 6F, a chemical-mechanical polishing (CMP) process maybe used to planarize the surface of the layer of dielectric material 54and to expose a selected portion of the tip 78 on the end 77 of thecatalytic structure 76 therethrough. As shown in FIG. 6G, the exposedtip 78 on the end 77 of the catalytic structure 76 may be used tocatalyze formation or growth of a single nanowire 22 thereon aspreviously described with reference to FIG. 2G.

Referring to FIG. 6H, another layer of dielectric material 56 then maybe deposited over the workpiece and around the nanowire 22, and achemical-mechanical polishing (CMP) process may be used to planarize thelayer of dielectric material 56 and expose the first end 24 of thenanowire 22 therethrough. As shown in FIG. 6I, a volume of variableresistance material 20 and a second electrode 18 then may be formed onthe workpiece over the first end 24 of the nanowire 22 in the mannerpreviously described with reference to FIGS. 2G-2I.

A sixth embodiment of a method that may be used to form a memory device10 like that shown in FIG. 1A is described below with reference to FIGS.7A-7I. Referring to FIG. 7A, a workpiece may be provided that includesthe substrate 11 and a conductive pad 28. A layer of catalyst material68 may be deposited over the substrate 11. By way of example and notlimitation, the layer of catalyst material 68 may be deposited using aphysical vapor deposition (PVD) process (e.g., sputtering or thermalevaporation), a chemical vapor deposition (CVD) process, an electrolessdeposition process, or by an electroless deposition process used to forma seed layer followed by a subsequent electroplating process. The layerof catalyst material 68 may have an average thickness of between aboutthirty nanometers (30 nm) and about two hundred nanometers (200 nm). Amask layer 70 then may be provided over the layer of catalyst material68. The mask layer 70 may comprise, for example, a layer of nitridematerial (e.g., Si₃N₄) or a layer of oxide material (e.g., SiO₂). Themask layer 70 then may be selectively patterned to provide a discreteregion 72 of mask material on the surface of the layer of catalystmaterial 68 over the conductive pad 28. By way of example and notlimitation, the discrete region 72 of mask material may be generallycircular and may have an average diameter of between about thirtynanometers (30 nm) and about one hundred nanometers (100 nm).

Referring to FIG. 7C, a partially isotropic etching process (e.g., a wetchemical etch or a partially isotropic reactive ion etch (RIE)) then maybe used to remove the regions of the layer of catalyst material 68 thatare not covered or otherwise protected by the discrete region 72 of maskmaterial so as to form a catalytic structure 86. In other words, only aportion of the layer of catalyst material 68 vertically under thediscrete region 72 of mask material may remain after the partiallyisotropic etching process. The partially isotropic etching process mayresult in undercutting of the layer of catalyst material 68 below thediscrete region 72 of mask material, and the lateral sidewalls of theremaining catalyst material 68 may have a generally curved frustoconicalshape, as opposed to being substantially vertical, as shown in FIG. 7C.As previously discussed, the discrete region 72 of mask material may begenerally circular, and the resulting catalytic structure 86 may have agenerally frustoconical shape similar to a portion of a cone. The upperend 87 of the catalytic structure 86 may have a substantially circularcross-sectional shape having an average diameter less than the averagediameter of the discrete region 72 of mask material.

As shown in FIG. 7D, optionally, the exterior surfaces of the catalyticstructure 86 may be oxidized to form an oxidation layer 90 therein,which may effectively reduce the cross-sectional area of the catalyticstructure 86. The exterior surfaces of the catalytic structure 86 may beoxidized to form the oxidation layer 90 by, for example, heating theworkpiece in an oxidizing atmosphere. By selectively controlling theoxidation process so as to oxidize the exterior surfaces of thecatalytic structure 86 to a predetermined depth and, hence, provide apredetermined thickness of the oxidation layer 90, a selected effectivecross-sectional area of the catalytic structure 86 may be provided thatis less than the original cross-sectional area of the catalyticstructure 86. Furthermore, the effective cross-sectional area of thecatalytic structure 86 may be selected so as to facilitate growth of asingle nanowire 22 thereon.

Referring to FIG. 7E, the discrete region 72 of mask material remainingon the end of the catalytic structure 86 may be removed from the end ofthe catalytic structure 86 using, for example, a wet chemical etchingprocess. A substantially conformal layer of dielectric material 54 thenmay be deposited over the workpiece and around the catalytic structure86. The layer of dielectric material 54 may have an average thicknessthat is greater than a distance by which the catalytic structure 86extends from the surface of the conductive pad 28 and the substrate 11,as shown in FIG. 7E.

As shown in FIG. 7F, a chemical-mechanical polishing (CMP) process maybe used to planarize the surface of the layer of dielectric material 54and to expose a selected portion of the tip 88 on the end 87 of thecatalytic structure 86 therethrough. As shown in FIG. 7G, the exposedtip 88 on the end 87 of the catalytic structure 86 may be used tocatalyze formation or growth of a single nanowire 22 thereon, aspreviously described with reference to FIG. 2G.

Referring to FIG. 7H, another layer of dielectric material 56 may bedeposited over the workpiece and around the nanowire 22, and achemical-mechanical polishing (CMP) process may be used to planarize thelayer of dielectric material 56 and expose the first end 24 of thenanowire 22 therethrough. As shown in FIG. 7I, a volume of variableresistance material 20 and a second electrode 18 then may be formed onthe workpiece over the first end 24 of the nanowire 22 in the mannerpreviously described with reference to FIGS. 2G-2I.

A seventh embodiment of a method that may be used to form a memorydevice 10 like that shown in FIG. 1A is described below with referenceto FIGS. 8A-8E. Referring to FIG. 8A, a workpiece may be provided thatis substantially similar to that shown in FIG. 7D and includes acatalytic structure 86 having an oxidation layer 90 therein toeffectively reduce the cross-sectional area of the catalytic structure86. A discrete region 72 of mask material may remain over the catalyticstructure 86 as previously described.

Referring to FIG. 8B, the discrete region 72 of mask material remainingon the end of the catalytic structure 86 may be removed from the end ofthe catalytic structure 86 using, for example, a wet chemical etchingprocess. As shown in FIG. 8C, the exposed tip 88 on the end of thecatalytic structure 86 then may be used to catalyze formation or growthof a single nanowire 22 thereon, as previously described with referenceto FIG. 2G.

Referring to FIG. 8D, a substantially conformal layer of dielectricmaterial 54 then may be deposited over the workpiece and around thecatalytic structure 86 and the nanowire 22. The layer of dielectricmaterial 54 may have an average thickness that is greater than adistance by which the catalytic structure 86 and the nanowire 22 extendfrom the surface of the conductive pad 28 and the substrate 11. Achemical-mechanical polishing (CMP) process may be used to planarize thesurface of the layer of dielectric material 54 and to expose the firstend 24 of the nanowire 22 therethrough.

As shown in FIG. 8E, a volume of variable resistance material 20 and asecond electrode 18 then may be formed on the workpiece over the firstend 24 (FIG. 8D) of the nanowire 22 in the manner previously describedwith reference to FIGS. 2G-2I.

Memory devices like that shown in FIG. 1A may be used in embodiments ofelectronic systems of the present invention. For example, FIG. 9 is ablock diagram of an illustrative electronic system 100 according to thepresent invention. The electronic system 100 may comprise, for example,a computer or computer hardware component, a server or other networkinghardware component, a cellular telephone, a digital camera, a personaldigital assistant (PDAs), portable media (e.g., music) player, etc. Theelectronic system 100 includes at least one memory device of the presentinvention, such as the embodiment of the memory device 10 shown in FIG.1A. The system 100 further may include at least one electronic signalprocessor device 102 (often referred to as a “microprocessor”). Theelectronic system 100 may, optionally, further include one or more inputdevices 104 for inputting information into the electronic system 100 bya user, such as, for example, a mouse or other pointing device, akeyboard, a touchpad, a button, or a control panel. The electronicsystem 100 may further include one or more output devices 106 foroutputting information (e.g., visual or audio output) to a user such as,for example, a monitor, display, printer, speaker, etc. The one or moreinput devices 104 and output devices 106 may communicate electricallywith at least one of the memory device 10 and the electronic signalprocessor device 102.

While the present invention has been described in terms of certainillustrated embodiments and variations thereof, it will be understoodand appreciated by those of ordinary skill in the art that the inventionis not so limited. Rather, additions, deletions and modifications to theillustrated embodiments may be effected without departing from thespirit and scope of the invention as defined by the claims that follow.

1. A memory device comprising at least one memory cell having an anode,a cathode, and a volume of variable resistance material disposed betweenthe anode and the cathode, at least one of the anode and the cathodecomprising a single nanowire having one end thereof in electricalcontact with the variable resistance material, wherein the at least oneof the anode and the cathode comprising the single nanowire furthercomprises a generally conical-shaped conductive catalytic structurecomprising a catalyst material with the single nanowire formed on a tipthereof.
 2. The memory device of claim 1, wherein the at least one ofthe anode and the cathode comprising the single nanowire furthercomprises a conductive pad, the nanowire providing electrical contactbetween the volume of variable resistance material and the conductivepad.
 3. The memory device of claim 1, wherein the nanowire comprises acarbon nanotube having at least one wall.
 4. The memory device of claim1, wherein the nanowire comprises at least one of silicon, germanium,gallium, a III-V type semiconductor material, a II-VI type semiconductormaterial, and a metal.
 5. The memory device of claim 1, wherein thenanowire comprises at least one of a superlattice structure and a PNjunction.
 6. The memory device of claim 1, wherein the variableresistance material comprises a phase change material.
 7. The memorydevice of claim 1, wherein the catalyst material comprises at least oneof aluminum, cobalt, gallium, gold, indium, iron, molybdenum, nickel,palladium, platinum, silver, tantalum, and zinc.
 8. The memory device ofclaim 1, wherein the generally conical-shaped conductive catalyticstructure includes a base in electrical contact with a conductive padand wherein the tip is in electrical contact with a second end of thesingle nanowire opposite a first end of the single nanowire inelectrical contact with the variable resistance material.
 9. The memorydevice of claim 8, wherein the tip has a minimum cross-sectional area ofless than about 300 square nanometers.
 10. The memory device of claim 1,further comprising a layer of dielectric material on at least a portionof an exterior surface of the conductive catalytic structure.
 11. Amemory device having at least one memory cell comprising a variableresistance material disposed between an anode and a cathode, at leastone of the anode and the cathode comprising a single nanowire formed ona tip of a generally conical-shaped conductive catalytic structuredisposed between a conductive pad and the single nanowire, the singlenanowire providing electrical contact between the conductive pad and thevariable resistance material, wherein the memory cell is configured tocause current flow between the anode and the cathode at leastpredominantly through the single nanowire in response to a voltageapplied between the anode and the cathode.
 12. The memory device ofclaim 11, wherein the single nanowire provides at least a portion of asole, low-resistance electrical pathway between the variable resistancematerial and the conductive pad within the memory cell.
 13. The memorydevice of claim 11, wherein the single nanowire comprises a carbonnanotube having at least one wall.
 14. The memory device of claim 11,wherein the variable resistance material comprises a phase changematerial.
 15. The memory device of claim 11, wherein the generallyconical-shaped catalytic structure includes a base electrically coupledto the conductive pad and wherein the tip is electrically coupled to thesingle nanowire.
 16. The memory device of claim 15, further comprising adielectric material on at least a portion of an exterior surface of thegenerally conical-shaped conductive catalytic structure.
 17. Anelectronic system comprising: at least one electronic signal processor;at least one memory device configured to communicate electrically withthe at least one electronic signal processor, the at least one memorydevice comprising at least one memory cell having an anode, a cathode,and a volume of variable resistance material disposed between the anodeand the cathode, at least one of the anode and the cathode comprising: asingle nanowire having one end thereof in electrical contact with thevariable resistance material; and a conductive catalytic structurecomprising a catalyst material and disposed between a conductive pad andthe single nanowire, wherein the conductive catalytic structure is agenerally conical structure including a tip and a base; and at least oneof an input device and an output device configured to communicateelectrically with the at least one electronic signal processor.
 18. Theelectronic system of claim 17, wherein the at least one of the anode andthe cathode comprising the single nanowire further comprises aconductive pad, the nanowire providing electrical contact between thevolume of variable resistance material and the conductive pad.
 19. Theelectronic system of claim 17, wherein the variable resistance materialcomprises a phase change material.
 20. The electronic system of claim17, wherein the catalyst material comprises at least one of aluminum,cobalt, gallium, gold, indium, iron, molybdenum, nickel, palladium,platinum, silver, tantalum, and zinc.
 21. The electronic system of claim17, wherein the base is electrically coupled to the conductive pad andthe tip is electrically coupled to an end of the single nanowireopposite the variable resistance material.